tools: bpftool: fix a bitfield pretty print issue

Commit b12d6ec09730 ("bpf: btf: add btf print functionality") added btf pretty print functionality to bpftool. There is a problem though in printing a bitfield whose type has modifiers.

For example, for a type like
typedef int ___int;

  struct tmp_t {
          int a:3;
          ___int b:3;

};
Suppose we have a map
struct bpf_map_def SEC("maps") tmpmap = {
.type = BPF_MAP_TYPE_HASH, .key_size = sizeof(__u32), .value_size = sizeof(struct tmp_t), .max_entries = 1,
};
and the hash table is populated with one element with key 0 and value (.a = 1 and .b = 2).

In BTF, the struct member "b" will have a type "typedef" which points to an int type. The current implementation does not pass the bit offset during transition from typedef to int type, hence incorrectly print the value as

  $ bpftool m d id 79
  [{
          "key": 0,
          "value": {
              "a": 0x1,
              "b": 0x1
          }
      }
  ]

This patch fixed the issue by carrying bit_offset along the type chain during bit_field print. The correct result can be printed as

  $ bpftool m d id 76
  [{
          "key": 0,
          "value": {
              "a": 0x1,
              "b": 0x2
          }
      }
  ]

The kernel pretty print is implemented correctly and does not have this issue.

Fixes b12d6ec09730 ("bpf: btf: add btf print functionality")

Signed-off-by Yonghong Song <[email protected]>

---
 tools/bpf/bpftool/btf_dumper.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

tools/bpf/bpftool/btf_dumper.c +3 -4

index 55bc512a1831..e4e6e2b3fd84 100644
@@ -32,7 +32,7 @@ static void btf_dumper_ptr(const void *data, json_writer_t *jw,
 }
 
 static int btf_dumper_modifier(const struct btf_dumper *d, __u32 type_id,
-                              const void *data)
+                              __u8 bit_offset, const void *data)
 {
        int actual_type_id;
 

@@ -40,7 +40,7 @@ static int btf_dumper_modifier(const struct btf_dumper *d, __u32 type_id,
        if (actual_type_id < 0)
                return actual_type_id;
 
-       return btf_dumper_do_type(d, actual_type_id, 0, data);
+       return btf_dumper_do_type(d, actual_type_id, bit_offset, data);
 }
 
 static void btf_dumper_enum(const void *data, json_writer_t *jw)

@@ -237,7 +237,7 @@ static int btf_dumper_do_type(const struct btf_dumper *d, __u32 type_id,
        case BTF_KIND_VOLATILE:
        case BTF_KIND_CONST:
        case BTF_KIND_RESTRICT:
-               return btf_dumper_modifier(d, type_id, data);
+               return btf_dumper_modifier(d, type_id, bit_offset, data);
        default:
                jsonw_printf(d->jw, "(unsupported-kind");
                return -EINVAL;
--

2.17.1

Commit b12d6ec09730 ("bpf: btf: add btf print functionality") added btf pretty print functionality to bpftool. There is a problem though in printing a bitfield whose type has modifiers.

For example, for a type like
typedef int ___int;

  struct tmp_t {
          int a:3;
          ___int b:3;

};
Suppose we have a map
struct bpf_map_def SEC("maps") tmpmap = {
.type = BPF_MAP_TYPE_HASH, .key_size = sizeof(__u32), .value_size = sizeof(struct tmp_t), .max_entries = 1,
};
and the hash table is populated with one element with key 0 and value (.a = 1 and .b = 2).

In BTF, the struct member "b" will have a type "typedef" which points to an int type. The current implementation does not pass the bit offset during transition from typedef to int type, hence incorrectly print the value as

  $ bpftool m d id 79
  [{
          "key": 0,
          "value": {
              "a": 0x1,
              "b": 0x1
          }
      }
  ]

This patch fixed the issue by carrying bit_offset along the type chain during bit_field print. The correct result can be printed as

  $ bpftool m d id 76
  [{
          "key": 0,
          "value": {
              "a": 0x1,
              "b": 0x2
          }
      }
  ]

The kernel pretty print is implemented correctly and does not have this issue.

Fixes b12d6ec09730 ("bpf: btf: add btf print functionality")

Signed-off-by Yonghong Song <[email protected]>